1. Field of the Invention
The present invention relates to a DC power source apparatus, and particularly, to a power converting transformer for a DC power source apparatus.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a DC power source apparatus according to a related art. In the apparatus of FIG. 1, a DC power source E supplies a DC voltage. A switching element Q1 is, for example, a MOSFET and carries out ON/OFF operations to convert the DC voltage from the DC power source E into high-frequency power. A transformer 1a is connected between the switching element Q1 and a positive electrode of the DC power source E, so that the high-frequency power is transmitted from the primary side of the transformer 1a to the secondary side thereof. The high-frequency power on the secondary side of the transformer 1a is converted into a DC output voltage through a rectifying/smoothing circuit consisting of a diode D1 and a smoothing capacitor C1. The DC output voltage is supplied to a load. An output voltage detector 3 detects the DC output voltage, compares the detected voltage with a reference voltage, and provides an error signal representative of the result of the comparison. According to the error signal, a control circuit (controller) 5 controls ON/OFF intervals of the switching element Q1 so that a predetermined output voltage is supplied to the load.
A tertiary winding D of the transformer 1a induces a voltage, which is rectified and smoothed through a diode D2 and a capacitor C2. The rectified and smoothed voltage is supplied as a source voltage to the controller 5.
FIG. 2 is a sectional view showing the transformer 1a in the DC power source apparatus of FIG. 1, FIG. 3 is a view showing windings of the transformer 1a, and FIG. 4 is a sectional view showing the transformer 1a and parasitic capacitance formed among the windings of the transformer 1a. 
In FIG. 2, the transformer 1a has a core 11 made of magnetic material inserted into a bobbin 13. Inside the bobbin 13, first primary winding P1, a secondary winding S, a second primary winding P2, and the tertiary winding D are sequentially are sequentially arranged. The first primary winding P1 consists of two winding layers P1-1 and P1-2. The second primary winding P2 consists of two winding layers P2-1 and P2-2.
Forming of the windings in the bobbin 13 will be explained. A wire is wound from a right end of the bobbin 13 in a vertical downward direction to form the winding layer P1-1. The wire is turned at a left end of the bobbin 13 and is wound to form the winding layer P1-2 on the winding layer P1-1, thereby completing the first primary winding P1. On the winding layer P1-2, the secondary winding S is wound. Thereafter, the winding layers P2-1 and P2-2 are wound in the same direction as the winding layers P1-1 and P1-2.
To improve manufacturability, the windings of the transformer 1a are usually wound in the same direction. In FIG. 1, the first and second primary windings P1 and P2 are connected in parallel. In FIGS. 2 and 4, the secondary winding S is arranged between the first and second primary windings P1 and P2, to increase the degree of coupling of these windings P1, P2, and S. In this case, there is parasitic capacitance C112 between the winding layers P1-1 and P1-2, parasitic capacitance C12S between the winding layer P1-2 and the secondary winding S, parasitic capacitance C21S between the secondary winding S and the winding layer P2-1, and parasitic capacitance C212 between the winding layers P2-1 and P2-2 produced.
In FIGS. 1 and 2, the winding layer P1-1 of the first primary winding P1 and the winding layer P2-1 of the second primary winding P2 adjacent to the secondary winding S are on the switching element Q1 side.
The switching element Q1 is continuously turned on and off therefore, the potential thereof greatly varies for the ON/OFF operations. The potential variations of the switching element Q1 are applied to the first and second primary windings P1 and P2 of the transformer 1a. As a result, high-frequency currents pass through the parasitic capacitance C12S between the winding layer P1-2 of the first primary winding P1 and the secondary winding S and the parasitic capacitance C21S between the winding layer P2-1 of the second primary winding P2 and the secondary winding S to the secondary side of the transformer 1a. 
Such high-frequency currents pass through a loop consisting of the first and second primary windings P1 and P2, the secondary winding S, a circuitry on the secondary side, the ground, the parasitic capacitance between the ground and a circuitry on the primary side, the circuitry on the primary side, and the first and second primary windings P1 and P2. Passing to the ground, the high-frequency currents cause common-mode noise. The common-mode noise leaks to the DC power source side and is radiated into space to badly affect other devices.
When the switching element Q1 is turned on, the DC voltage from the DC power source E is applied to a negative side of the first and second primary windings P1 and P2 of the transformer 1a. When the switching element Q1 is turned off, a flyback voltage occurs on a positive side of the first and second primary windings P1 and P2. Namely, first terminals of the first and second primary windings P1 and P2 connected to the switching element Q1 are subjected to large potential variations, and second terminals thereof connected to the DC input voltage that is stable are subjected to no potential variation.
The parasitic capacitance between the first and second primary windings P1 and P2 and the secondary winding S increases as the distance between them shortens. Accordingly, the high-frequency currents passing through the parasitic capacitance between the first and second primary windings P1 and P2 and the secondary winding Swill be large if the first terminals of the first and second primary windings P1 and P2 connected to the switching element Q1 are close to the secondary winding S.
In FIG. 4, the start of the first and second primary windings P1 and P2 are connected to the switching element Q1. The winding layer P1-1 that is at the start of the first primary winding P1 is located away from the secondary winding S, and the winding layer P2-1 that is at the start of the second primary winding P2 is located adjacent to the secondary winding S. Accordingly, a large high-frequency current passes the second primary winding P2 through the parasitic capacitance C21S to the secondary winding S. In FIGS. 1 and 4, an arrow represents a high-frequency current with the width of the arrow indicating the magnitude of the current.
To reduce the common-mode noise caused by high-frequency currents, FIGS. 5 to 7 show a transformer 1b according to another related art. FIG. 5 is a sectional view showing the structure of the transformer 1b, FIG. 6 is a sectional view showing parasitic capacitance among windings of the transformer 1b, and FIG. 7 is a circuit diagram showing a DC power source apparatus employing the transformer 1b. 
The transformer 1b shown in FIGS. 5 to 7 has a shield plate 17 between a winding layer P2-1 of a second primary winding P2 and a secondary winding S, to reduce parasitic capacitance C21S between the winding layer P2-1 and the secondary winding S. Reducing the parasitic capacitance C21S results in reducing a high-frequency current passing from the primary side of the transformer to the secondary side thereof, thereby decreasing the common-mode noise.